The present invention relates to hitless protection for switching systems.
Ensuring no traffic loss becomes more and more important in today's telecommunication systems. Internet trading is one of the applications that require packet loss to be as low as possible; real-time Internet applications such as voice over IP and video conferences are coming to people's daily life, and packet loss is undesirable for causing of flickering noises on the phone lines or viewing distorted video clips. Service continuity is more critical comparing to traffic loss, so redundancy protection are always built into the tele-communications systems to avoid service interruption during system or link failure. Redundancy protection is the technique to provide a backup line card or fabric card and replace the primary one when failure occurs. The minimization of traffic loss when switching from the primary to the backup line card becomes important for the above application reasons.
Conventional communication systems typically have cell or packet based switch fabric and TDM (time-division multiplexing) line card(s) that switch the TDM traffic through the switch fabric to its destination line card(s). The TDM line card provides 1+1 card protection, where the protecting card processes traffic the same way as the working card. Hitless protection relates to the ability for the system to enter a protected mode without losing frame or framing synchronization when failure occurs, to ensure that the telecommunication equipment provides uninterrupted or continuous service and maintain an extremely high-reliability rating. This means no traffic loss or duplication is allowed when switched from primary to backup card. However, because of the delay uncertainty of traffic switching and failure reaction, traffic might encounter loss or corruption when failure happens.
Optical Transport Network (OTN) is believed to be the main solution for next generation optical core networks. It is composed of a set of Optical Network Elements connected by optical fiber links, able to provide functionality of transport, multiplexing, routing, management, supervision and survivability of optical channels carrying client signals. A distinguishing characteristic of the OTN is its provision of transport for any digital signal independent of client-specific aspects, i.e. client independence. ODU is the basic optical channel unit carried in OTN frame, organized in TDM pattern to provide both TDM and packet services. Several different rates of ODUs are defined, including ODU0˜ODU4, ODUflex, and ODU2e etc., for different capacity. ODU multiplexing is defined for all the hierarchies, including mapping from ODU0 into ODU4. Telecommunication carriers are expecting next generation integrated optical core switching node to provide multi-service and all levels of ODU granularities.
FIG. 1 shows an exemplary conventional system 100. This system has client line cards to provide client interface 118, such as line card 102 and 104; core interface (TDM) line card with 1+1 protection to provide core connection 116, such line card 108 as primary and 110 as its backup; and fabric card 106 for switching (and 118 for switch fabric backup). For output traffic to core interface 116, module 112 works as a switch (further controlled by protection control signal 114) to select output from either primary or backup interface; for incoming traffic from core link 116, module 112 replicates (for example, through a signal splitter) the inputs to both primary and backup line cards.